Ion-exchange and other glass-strengthening processes can create stresses in the glass and give rise to optical birefringence. Compressive and tensile stress regions formed in the glass can have relatively small depths (e.g., 10 micrometers) or relatively large depths (e.g., a few or many millimeters). The stress profile of a glass part can be deduced from the measurement of the refractive index profiles for orthogonal polarizations of measurement light, with the difference in the index profiles for both polarization states representing the birefringence. The stress is related to the birefringence through the stress-optic coefficient. The character (profile) of the stress can be quite complex and depends on a number of factors, including the base glass material, the glass production processes and the down-stream manufacturing process.
The stresses in a glass part can be altered by various external forces, including how the glass part is mounted into a device and how the glass part is used. During reliability and failure-mode studies, various quality-control procedures are used to test and measure the change in stress across the edges and throughout the volume of the glass part.
One technique that has been used to measure refractive index and birefringence profiles (and thus stress) in ion-exchanged, planar optical waveguides is the refracted near-field (RNF) method. The RNF method utilizes a system wherein a reference block is in contact with the glass part being measured. Measurement light passes through the glass part and the reference block and is detected by a photodetector arranged very close to the top of the reference block. An example RNF system is disclosed in U.S. Pat. No. 5,280,334.
A problem with prior-art RNF systems is that the photodetector, besides detecting the measurement light, also detects scattered and multiply reflected light from many angles and locations. The scattered light may arise from imperfections in an index fluid (e.g., contaminants), on the surfaces of the reference block (e.g., dust leftover after sub-optimal cleaning), in the optics of the RNF system or even within the glass sample itself (e.g., bubbles and other “seeds”). Such scattered light may lead to erroneous measurements of the refracted power of the glass part.